1. Field of the Invention
The present invention relates to improvements in dual compatibility computers and involves a method for allowing operation of multiple incompatible operating systems upon different disk partitions in the same hard disk drive.
2. Description of the Related Art
The IBM AT compatible computer, which uses an Intel 80.times.86 microprocessor is one of the most common standards for desk top computers in the United States. Microprocessors that are 80.times.86 compatible are also used in computers in Japan. For example, the NEC 9801 computer utilizes such an 80.times.86 compatible microprocessor. Many compatibilities exist between the IBM AT compatible computer standard and the NEC 9801 computer. These compatibilities make it possible to develop a double-compatible computer. However, numerous incompatibilities exist, particularly involving Input/Output (I/O) device addresses, I/O busses, peripheral devices, the Basic Input Output System (BIOS) used to manipulate hardware, and the operating systems. Some hardware incompatibilities can be solved by using hardware which is mode selectable. However, in many cases, this requires redundant hardware, each set operating in one mode or the other. This is often impractical and undesirable.
Neither the IBM AT compatible nor the NEC 9801 compatible operating systems allow for divisions (called partitions in the AT compatible systems and volumes in the NEC 9801 compatible systems) of the hard disk over and above the normal divisions used by the respective operating systems. A partition table holds information regarding the disk partitions. However, no provision was made in either the IBM AT or the NEC 9801 systems for booting up under a different hardware or non-compatible software configuration. The IBM AT compatible operating system originally had partition table definitions for DOS-12, DOS-16 and Extended DOS partitions and has been extended to allow other operating systems such as XENIX, Novell, CP/M-86, PCIX and others. However, these operating systems were programmed to be compatible only with the IBM AT operating system hard disk partition tables.
The NEC 9801 operating system was not programmed to be compatible with the IBM AT system, and the partition table and the Initial Program Load (IPL) boot loader of the NEC 9801 is incompatible with the partition table and IPL boot loader of the IBM AT. Moreover, each operating system assumes that its IPL and partition tables are located at absolute sector zero on the hard disk. Therefore, the IBM AT system does not provide a definition to identify that the NEC 9801 system is installed somewhere on the hard disk. If only one hard disk is used in the dual-compatible computer, then installing both operating systems will not be possible because if both systems are installed on the same hard disk then the IBM AT compatible operating system will write to the disk space occupied by the NEC partition because nothing indicates to the IBM AT system that the NEC operating system is present on the hard disk.
Likewise, the NEC operating system assumes that it has use of absolute sector zero on the hard disk and would overwrite AT operating system disk space.
Installing each operating system on a different hard disk drive provides a solution, but this requires redundant hardware. As explained, this is uneconomical and undesirable.
Tomcat Company, a Japanese corporation, attempted to solve this problem by making an assumption that the NEC 9801 compatible operating system is installed after the last AT partition on the hard disk and that no other disk partitions will follow the NEC 9801 system partition. This assumption solves the problem of the IBM AT system accessing NEC 9801 hard disk space. Once this assumption is made, then the NEC 9801 basic I/O system (BIOS) can reference all disk access calls past the last AT partition and the AT system can limit disk usage to the defined AT partition space, which precedes the NEC 9801 disk space. However, if the user attempts to create another AT partition, this would overwrite the NEC 9801 data, causing a complete loss of NEC 9801 data without warning.
Accordingly, a reasonable solution to the problem would utilize a method to operate both systems without making the assumption that the user will not change the partitioning of the hard disk.
Implementing a solution should allow installation and booting of two absolute zero sector IPL code segment and accompanying partition tables on the same disk. A solution to this problem would dramatically improve the feasibility of the IBM AT/NEC 9801 dual-compatible computers.